That is a totally excellent idea, everyone has surely had the problem of trying to get over the $25 mark for free shipping. You should promote that on cashback forums where people like a bargain.
I agree, it is a superb idea. It's pretty boring and painful to look at, so I'd definitely add a theme. If you wanted to monetize it, I doubt any viewers would have issues with that. I'm even going to bookmark it for next time I need to get free shipping. You could probably also promote some sort of coupon/cashback/free stuff site or product on that, since it's relevant.
Great idea! Now, go and get some traffic and SEO your site. It is time to make more money from your amazon commissions.
I am suggest to you that to fill up the space of your website so it will become a better... Have a good time